1. Field of the Invention
The present invention relates to a method of crystallizing amorphous silicon for forming large grain.
2. Description of the Related Art
Development of a recent telecommunication technology accelerates an information-intensive society of the future, and thus, takes an attention for personal information security. An existing security system such as secret number, key, or card for the personal information security as above has disadvantages of easy leakage to others or losing. Hence, in order to protect information more efficiently, personal identification methods using various physiological characteristics are applied.
Among the various physiological characteristics, one widely-used physiological characteristics is a fingerprint. Further, the identification method may be classified into an optical method, a semiconductor method and the like depending on its using tool. In recent years, the methods are now developed to fabricate thin flat-type systems by employing a new technology in a trend of development of telecommunication devices which become small-sized, low-priced, and modularized.
A thin film transistor (TFT) having a thin film structure as above is used as an essential technology for a fingerprint sensor and the like. Generally, the TFT forms its channel as a path for electrons by forming an amorphous silicon layer and crystallizing the silicon layer. In order to fabricate a device having a high operation speed by increasing a mobility of electrons, efforts of enlarging a size of grain in the crystallization of the amorphous silicon layer have been made.
Methods of delaying a crystallization speed to form large grain have been proposed. The crystallization speed is a ratio of a melting duration time to a melting depth, and if a melting duration time is relatively long compared to a predetermined melting depth, the crystallization speed is delayed so as to provide larger grain. Normally, if a crystallization speed is 1 m/s or lower when a melting duration time is 100 ns for a melting depth of 50 through 100 nm, it is known that a possibility of forming large grain is further increased.
A typical method of forming an amorphous silicon thin film to a polycrystal silicon thin film proposed to form large grain is to use an excimer pulse laser (308 nm). Currently, as a method of forming large grain using an excimer laser, there is a method of employing dual pulse laser using single pulse laser from a high laser pulse energy of an optical system. This provides large grain having a size of 5 μm at maximum. The method needs an expensive excimer laser, and a structure of its related optical system is complicated.
For example, the method discloses enlarging silicon grain further in size at a surface portion of the silicon grain than that of a normal silicon grain by delaying a crystallization speed using dual pulse laser. The method provides crystallizing silicon in a stack structure including amorphous silicon/oxide layer/metal using a dual excimer pulse laser. Silicon grain having a size of 5 μm can be formed from a bump structure by the method so as to form large silicon at a specific position (see A. Burtsev, Enlargement of location controlled Si grains by dual beam excimer laser with bump structures, Applied Surface Science). Referring to FIG. 1, amorphous silicon is crystallized by a high temperature process using the method as above so as to form large grain.
However, the method has a problem that high-priced optical systems are inevitably necessary to form double pulse laser. Further, the method has problems that used metal cannot be endurable at a high temperature and is broken during the crystallization, and one process must be selected initially depending on a top gate-type thin film transistor and a bottom gate-type thin film transistor.
Therefore, another method has been proposed to form large grain in order to solve the problems. The method provides large grain using a structure replacing a metal thin film, which is suitable to double pulse excimer laser, that is, is endurable to a high temperature. According to the method, silicon large grain having a size of 5 μm can be formed (see Ryoichi ISHIHARA, Location control of large grain following excimer laser melting of Si thin films, Japanese Journal of Applied Science). However, the method cannot solve the problem of increasing production costs due to a high-priced optical system, which is necessary to use dual pulse laser.
As another example, there is disclosed a method of enlarging a size of silicon grain at a specific surface portion of the silicon grain further than that of a typical case by examining crystallization of the silicon grain in accordance with respective pulse widths of dual pulse laser beams and time intervals between respective pulses. However, the method just provides grain having a size of 0.8 μm (see Ryoichi ISHIHARA , A Novel double pulse excimer laser crystallization method of silicon thin films, Japanese Journal of Applied Physics).